Isocyanate terminated liquid prepolymers have been produced in the past and these prepolymers have been employed as one of the ingredients in two-package urethane casting resins. The other ingredient used to cure the propolymer contains active hydrogens such as hydroxyls or amines. These two package casting resin systems are particularly useful for potting, filling, embedding and encapsulating in the electrical and biomedical fields.
One particularly useful isocyanate terminated propolymer is the reaction product of 4,4'diphenylmethane diisocyanate, castor oil, and a polyoxypropylene glycol. This product has been prepared and successfully used commercially in the past. This composition, however, preferably must be held at a temperature above 40.degree. F. until it is employed, since it tends to crystallize and must be heated above its melting point to return it to the liquid state.
Although prepolymers of other isocyanate compositions with polyols have been prepared, the instant invention, however, is concerned with MDI based prepolymers because of their desirable properties. These particular prepolymers are less toxic than other isocyanate based prepolymers such as 2,4 tolylene diisocyanate (TDI). They also react rapidly with polyols and the like to produce elastomers which possess superior mechanical properties combined with excellent heat and hydrolytic stability.
MDI prepolymers with NCO/OH ratios in the range of 3-7, however, preferably must be held at a temperature above 40.degree. F. until they are employed, since they tend to crystallize and form a solid mass below this temperature.
It is, therefore, desirable to produce modified MDI prepolymers which not only possess the above mentioned desirable properties, but also are substantially non-crystallizing at temperatures below 40.degree. F. and may even be non-crystallizing at 0.degree. F.